In the prior art, thread-wound golf balls are prepared by forming a cover around a wound core by a compression molding technique. The compression molding technique uses a molding press having for example 40 to 60 cavities. Owing to variances of temperature and pressure at the molding press surface and variances of working precision of cavities, wound golf balls as finished by the compression molding technique have large variations of the outer diameter and roundness as compared with injection molded solid golf balls. As compared with the injection mold, the molding press used in the compression molding technique must be provided with a greater number of cavities and thus requires a greater investment, which becomes a bar against cost-effective manufacture.
For wound golf balls, the injection molding of the cover as in the case of solid golf balls is desired in order to improve the productivity and stabilize the quality of wound golf balls. Various attempts were made to this end.
(1) For example, JP-A 47873/1980 and JP-A 115270/1982 describe golf balls prepared by impregnating a wound core with a latex containing 30 to 60% by weight of ionomer resin solids for causing the latex solids to penetrate into the network structure of the thread rubber layer for thereby integrally joining the cover to the thread rubber layer.
(2) JP-A 112618/1986 discloses the preparation of a golf ball by wrapping a pair of half-caps preformed from thermoplastic film over a wound core, compression molding the caps to form a protective layer, and injection molding a cover over the protective layer. AS to the compression molding of thermoplastic film, it is described to heat at a temperature at least 10.degree. C. higher than the softening point of the caps and lower than 105.degree. C.and to clamp the mold under a force of 100 to 1,000 kg for 1/2 to 3 minutes, preferably within one minute.
(3) JP-A 112619/1986 discloses a method for preparing a golf ball involving the consecutive steps of wrapping a wound core with a resilient thermoplastic film, causing heat shrinkage of the film to form a protective layer of substantially uniform gage closely bonded to the wound core, and injecting molding a cover over the surface of the protective layer.
(4) JP-A 109879/1988 discloses a golf ball which is prepared by wrapping a wound core with a thermoplastic resin film of 50 to 300 .mu.m thick showing an area shrinkage factor of less than 10% when heated at a temperature of 100 to 180.degree. C., and injection molding the same material as the film as a cover stock to enclose the wound core with the cover stock. The film is fused by the thermal inertia of the cover stock to thereby integrate the film with the cover stock.
Of these attempts of injection molding the cover of wound golf balls, the method of impregnating the thread rubber layer with the ionomer latex and the method of wrapping the wound core with the thermoplastic film eliminate the possibility that the heat applied during injection molding can degrade the thread rubber or cause the thread rubber to be ruptured and exposed at the cover surface, but have the drawback that the durability comparable to compression molding is not achievable because it is difficult to infiltrate the cover material deeply into interstices of the thread rubber layer for enhancing the bond of the cover to the thread rubber layer.
The injection molding method (2) of JP-A 112618/1986 fails to cause the half-cap material to deeply penetrate the thread rubber layer because the temperature and time of compression molding of the half-caps onto the wound core is low and short, respectively. No firm bond is established between the cover and the wound core.